Production tanks typically are a source of hydrocarbon vapors emitting into the atmosphere. Various systems are available for collection oil and gas vapors. Recently, the Colorado Department of Health has adopted regulations limiting emissions from production tanks. Depending on temperature, color of production tank, orientation to the sun, and gravity of the contained liquids, coupled with the normal separator operations, the amount of vapors may vary from minimal to in excess of 4 mcfd. Typically these vapors have a very high BTU content. Capture and beneficial usage of these vapors, as opposed to flaring, is both economically and environmentally advantageous. The present system ensures that no air or oxygen leakage into the production tank occurs when gauging and/or emptying the tank, plus provides a constant reservoir-type storage system by utilizing a variable volume, at a constant pressure, to minimize the compressor cycling and simultaneously accommodating rapid influxes of liquids and/or vapors into the tank. Thus, when the separator dumps or the plunger lift system adds significant volumes of both volatile oil-condensate, as well as the associated highly volatile gases, the surge of gas is accommodated by the present system. The gas is temporarily stored in an expandable gas storage bag which accommodates hydrocarbons. The bag collapses, like an accordion, when very little gas is present, but expands rapidly, and under minimal pressure of water column pressure of approximately 1 to 2 inches, to contain the surge of gas. As the bag expands, the bag actuates a switch when turns a compressor on. The compressor, in turn, compresses the contained gas and into the pipeline. As the bag decompresses and shrinks, a switch is activated to turn off the compressor. Thus, the present system allows the oil storage tank to operate at a constant pressure, but the gas storage bag accommodates variable gas volume.